MONDAY, June 25 (HealthDay News) -- External stimulation during sleep can help strengthen memory, which, in turn, can help you learn, a new study reports.

Researchers from Northwestern University noted that such stimulation could reinforce what people have already learned, but doesn't help them gain new skills.

"The critical difference is that our research shows that memory is strengthened for something you've already learned," the study's co-author, Paul Reber, associate professor of psychology at Northwestern, said in a university news release. "Rather than learning something new in your sleep, we're talking about enhancing an existing memory by reactivating information recently acquired."

In conducting the study, researchers taught participants how to play two musical tunes by pressing certain keys at certain times. After they learned how to play the artificially generated tunes, the participants took a 90-minute nap. While they slept, only one of the songs was played. The soft musical cues, the researchers noted, were played during slow-wave sleep, a stage of sleep that is linked to storing memories.

As the participants napped, the researchers recorded their electrical brain activity using electroencephalography. After they woke up, the participants made fewer mistakes when playing the tune that was played while they were sleeping than the one that was not played.

"Our results extend prior research by showing that external stimulation during sleep can influence a complex skill," the study's senior author, Ken Paller, professor of psychology in the Weinberg College of Arts and Sciences at Northwestern, said in the news release.

"We also found that electrophysiological signals during sleep correlated with the extent to which memory improved," added lead author James Antony, of the Interdepartmental Neuroscience Program at Northwestern. "These signals may thus be measuring the brain events that produce memory improvement during sleep."

The researchers said they are investigating how their findings could possibly be applied to other types of learning, such as studying a foreign language. They noted their research could also lead to more studies on sleep-based memory-processing involving other types of skills, habits and behaviors.